High viscosity fluids are useful in many functions conducted in subterranean formations, particularly subterranean formations that are potential sources of oil and natural gas. Such functions can include, for example, hydraulic fracturing, gravel packing, matrix acidizing, and fracture acidizing.
Viscoelastic surfactants (VES) are a class of gelling agents that have been used to provide high viscosity fluids for many such functions in subterranean formations, particularly in the recovery of oil and gas. These high viscosity fluids, known as gels, can perform functions such as acidizing, fracturing, and completion. After the desired function has been performed, there is oftentimes a need to break the gel to reduce the viscosity of the fluid to allow its removal from one or more sites in the subterranean formation.
One option to break the gel is dilution with fluids naturally present in the formation, such as crude oil hydrocarbon, condensate and formation water. In many instances, however, VES fluids in the formation are not completely broken which can make their removal difficult and impede recovery of the desired subterranean oil or gas. One method to improve the clean-up of VES fluids is to use pre-flush or post-flush fluids which may contain aromatic hydrocarbons, alcohols, mutual solvents and/or other breaking additives. One problem with the use of pre-flush or post-flush fluids is their limited interaction with the gel due to small fluid-fluid interfaces. Usually only some sections of the formation achieve the fluid-fluid interface required to break the gel and allow its removal, while in the remaining sections of the formation the gel remains unbroken. Therefore, contacting and breaking the gel with pre-flush or post-flush treatment is not always effective. Additionally, the volumes of the flushes can be high.
To efficiently break a viscosified fluid and to have a better control of breaking, it is desirable to have a delayed internal breaker which breaker can be activated by subterranean conditions to provide a controlled rate of gel viscosity reduction.
Various types of delayed internal breakers are known to reduce the viscosity of VES gels, such as acids, oxides, enzymes, and transition metals. Proper placement is a key factor for any breaker. It must have a sufficient interface with the fluid that is to be broken. Most conventional breakers cannot clean up the VES gel once the high viscosity fluid enters the formation fissures or pores, because the required interface cannot be achieved. Therefore, there exists a need for compositions and methods for breaking VES fluids under different subterranean conditions and at predetermined times.
U.S. Pat. No. 6,881,709 B2 discloses a precursor of a breaking system which reduces the viscosity of the fluid by melting, slow dissolution, and de-adsorption of a breaking agent absorbed into solid particles. In one disclosed embodiment the breaker comprises salts provided in an encapsulated form. It is possible that such a solid breaker formulation may settle out of the gel and cause an inefficient or premature break of the VES gel. Moreover, if a solid encapsulated breaker formulation is used in hydraulic fracturing, the breaker mechanism may rely on the fracture closure to crush the capsules and release the breaking salt. In that case, any high viscosity fluid that is forced out of the fracture during the closure would not be broken. U.S. Pat. No. 7,084,095 discloses a method of treating a subterranean formation comprising the steps of injecting down a well an aqueous fluid comprising a thickening amount of a viscoelastic surfactant and providing an encapsulated polymer additive that causes a change in rheology of the fluid under downhole conditions. The patent also discloses a polymer additive for treatment of a subterranean formation wherein the polymer additive is selected from sulfonated polynaphthalenes, sulfonated polystyrenes, and sulfonated styrene/maleic anhydride polymers.
U.S. Pat. No. 7,635,028 discloses an acidic internal breaker, which can be an inorganic or organic acid. However, this approach may not be optimum in applications that are conducted at low pH. The reference additionally discloses that a corrosion inhibitor can be used with the acidic internal breaker to protect any well tubing that may be present.
U.S. Pat. No. 7,655,603 discloses a method for treating a subterranean reservoir comprising the use of an aqueous fluid gelled with a viscoelastic surfactant and a micro-emulsion, wherein the micro-emulsion comprises at least one viscosity reducing agent comprising at least one unsaturated fatty acid, at least one solubilizing agent, at least one desorption agent, and at least one water wetting agent.
U.S. Pat. No. 7,879,770 discloses an oxidative internal breaking agent and a free radical propagating agent selected from metabisulfites, reducing sugars and reducing di-, tri-, oligo- and poly-saccharides. The oxidative breaking agent can be triggered by chemical or physical conditions, such as temperature or pH. The breaking rate can be optionally accelerated or delayed. It is possible, however, that oxidizers may not always work efficiently as delayed breakers. Furthermore, it is possible that phase separation and/or precipitation may occur if a higher dosage is used.
Thus there is a need for a delayed internal breaking agent that can be mixed with a viscoelastic surfactant, yet does not reduce the viscosity and elasticity of the VES fluid for at least several hours so that the VES fluid can perform its function. Ideally the internal breaking agent should be compatible with the VES, should move throughout the formation along with the VES, and should reduce both the viscosity and elasticity of the VES fluid at the appropriate time.